Forced convective heat transfer and flow characteristics of ionic liquid as a new heat transfer fluid inside smooth and microfin tubes

Ionic liquids (ILs) have thermophysical and chemical properties that may be suitable for use as heat transfer fluids from the low to high temperatures. This paper presents the experimental results of forced convection heat transfer for 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4) IL, in microfin and smooth tubes. The horizontal test section is a counter flow double tube heat exchanger with IL flowing in the inner tube and cooling water flowing in the annulus. In addition to evaluating the forced convection heat transfer, thermophysical properties of [HMIM]BF4 such as thermal conductivity, density, heat capacity, and viscosity were also experimentally measured over the temperature range of 303-453 K. The correlations of thermophysical properties were fitted with measured data. The experimental results show that the frictional coefficient and Nusselt number inside the smooth tube agree with those predicted by the Hagen-Poiseuille and Sieder-Tate correlations. In the Reynolds number range of 57-538, the frictional coefficient and Nusselt Number of the microfin tube are 5.6% and 5.4-11.3% higher than those of the smooth tube, respectively. Based on the experimental data, the empirical correlations were developed to calculate the frictional coefficient and convective heat transfer coefficient for the microfin tube under laminar flow condition.